1. Field of the Invention
The present invention relates to an optical spectrum analyzer and an optical spectrum detecting method, and in particular to an optical spectrum analyzer and an optical spectrum detecting method having a function of monitoring optical power per wavelength of a wavelength-(division-)multiplexed optical transmission signal.
As an optical spectrum analyzer having such a function, one is known which is composed of a spectrograph and a photodevice array (e.g. Japanese Patent Application Laid-open No. 9-210783). In comparison with one having mechanical movable parts, such an optical spectrum analyzer is reliable, so that it has been of greater importance as one preferably applied to an optical transmission apparatus required to be used for a long term.
2. Description of the Related Art
FIG. 9 shows a prior art optical spectrum analyzer having no mechanical movable parts as mentioned above. In FIG. 9, an output light from an optical fiber 1 is separated into P polarization and S polarization by a polarization compensating plate 2, thereby suppressing a polarization dependence. Both polarizations are sent, through a lens 3, to a diffraction grating 4 serving as a spectrograph.
The polarizations are spatially separated per wavelength component in a wavelength-multiplexed optical transmission signal by the diffraction grating 4, reflected by a reflection mirror 60 through a lens 5, and inputted to a photodevice array 7.
By such an arrangement, a wavelength-multiplexed signal light is separated per wavelength at the diffraction grating 4, passes through the reflection mirror 60 for the enhancement of measurement accuracy, whereby a longer path is formed. The signal light then enters into a photodevice array 7 composed of a plurality of photodevices (not shown) to which wavelengths are preliminarily assigned, so that the wavelength and the power of the entered signal light are outputted to be measured.
Since such a prior art optical spectrum analyzer has an arrangement of detecting the wavelength-multiplexed optical transmission signal by the photodevice array, it has been disadvantageous that resolutions of a wavelength and optical power are limited and a high accuracy measurement is difficult.
Namely, in a conventional wavelength-multiplexed optical communication system, for the wavelength resolution, the number of the photodevices assigned for detecting a single wavelength is physically limited to three or so, resulting in a problem that the measurement accuracy deteriorates in case where a center (peak) of an optical beam does not enter into the photodevice array in the wavelength measurement.
This will be described referring to an example shown in FIG. 1A. When the photodevice array 7 is composed of photodevices such as PD1-PD5, and an incident light from the reflection mirror 60 has a power distribution {circle around (1)}, the peak of the incident light is formed between the photodevices PD2 and PD3, whereby the center of the photodevice does not coincide with that of the optical beam. As a result, a wavelength xcex2 assigned to the adjoining photodevice PD2 is erroneously detected, although a wavelength xcex3 assigned to the photodevice PD3, for example, should be detected.
It is accordingly an object of the present invention to achieve an optical spectrum analyzer and an optical spectrum detecting method wherein the analyzer is composed of a spectrograph and a photodevice array, and consistently enters an optical beam, without increasing the number of the photodevices composing a photodevice array, into a center of each photodevice.
In order to achieve the above-mentioned object, an optical spectrum analyzer according to the present invention comprises: a spectrograph, an acoustooptic device for diffracting an output light of the spectrograph, a photodevice array for detecting a wavelength of a diffraction light or a non-diffraction light from the acoustooptic device, and a control circuit for detecting a wavelength deviation, from an assigned wavelength, of a light detected by the photodevice array to control a diffraction angle of the acoustooptic device.
Namely, in an optical spectrum analyzer according to the present invention, an acoustooptic device having a substance whose refractive index (diffraction angle) is changed by modulating an acoustic frequency is substituted for a reflection mirror 60 in the prior art optical spectrum analyzer shown in FIG. 9. A control circuit detects a wavelength deviation, from an assigned wavelength, of a light detected by a photodevice array and controls the diffraction angle of the acoustooptic device.
Thus, as shown in FIG. 1B, the wavelength is shifted from the state of a power distribution {circle around (1)} (corresponding to FIG. 1A) of an incident light in case where the acoustooptic device is not used, to the state of a power distribution {circle around (2)}. Accordingly, the center of the photodevice PD3, in this example, coincides with the peak of the incident light, so that the wavelength (and the power) of the incident light is measured as a wavelength xcex3 preliminarily assigned to the photodevice PD3.
The above-mentioned control circuit may be composed of a wavelength deviation detecting circuit for detecting wavelength deviations between wavelengths preliminarily assigned to photodevices composing the photodevice array and a wavelength of the light detected by the photodevice array, a beam diffraction angle calculator for calculating, from the wavelength deviation, a beam diffraction angle for providing incident light to the photodevice corresponding to the assigned wavelength, and an acoustic frequency calculating circuit for calculating an acoustic frequency from the beam diffraction angle to be provided to the acoustooptic device.
Also, the above-mentioned wavelength deviation detecting circuit may be composed of a calculator for calculating a peak wavelength of the light detected by the photodevice array, and a detector for detecting a wavelength deviation between the peak wavelength and a closest wavelength among the photodevices in the photodevice array.
Furthermore, the above-mentioned calculator may obtain an intensity of each photodevice to obtain a Gaussian distribution from the intensity, thereby calculating the peak wavelength.
Namely, if two photodevice arrays are provided for respectively receiving an exit light and a diffraction light from the acoustooptic device, and for mutually compensating gaps between photodevices, accurate wavelength detection can be performed by either of the photodevice arrays.
It is noted that as the above-mentioned acoustooptic device, either a reflection-type or a transmission-type may be used, whereby the wavelength detection can be performed at the photodevice array by using the exit light and/or the diffraction light.
It is to be noted that the above-mentioned optical spectrum analyzer may further include a polarization compensating plate for separating a wavelength-multiplexed input signal into orthogonal components.
Furthermore, as the above-mentioned spectrograph, a diffraction grating may be used which spacially separates an output light of the polarization compensating plate into each wavelength component.
Also, in the present invention, for achieving the above-mentioned object, an optical spectrum detecting method is provided which detects, when an output light of a spectrograph is detected by a photodevice array through an acoustooptic device, a wavelength deviation, from an assigned wavelength, of a light detected by the photodevice array, and controls a diffraction angle of the acoustooptic device.
The above-mentioned control of the diffraction angle may be performed by detecting wavelength deviations between wavelengths preliminarily assigned to photodevices composing the photodevice array and a wavelength of the light detected by the photodevice array, by calculating, from the wavelength deviation, a beam diffraction angle for providing incident light to the photodevice corresponding to the assigned wavelength, and by calculating an acoustic frequency from the beam diffraction angle to be provided to the acoustooptic device.
The above-mentioned wavelength deviation may be detected by calculating a peak wavelength of the light detected by the photodevice array, and by detecting a wavelength deviation between the peak wavelength and a closest wavelength among the photodevices in the photodevice array.
Also, the above-mentioned peak wavelength may be calculated by obtaining an intensity of each photodevice and by obtaining a Gaussian distribution from the intensity.